Field-Effect Flow Control in Microfluidics

Lab-on-chip (LOC) devices have miniaturized routine laboratory processes for automated, high-throughput chemical analysis. Separations of biomolecules, including protein and DNA, have been performed with high efficiencies in LOC devices, but the need for improved fluid flow control, i.e. pumping and valves, remains a significant challenge for next-generation systems. This dissertation explores the development of novel flow-control technology in polymer microfluidic networks for the realization of inexpensive, next-generation LOC devices. In the microchannels, electroosmotic flow (EOF) is used to electro-kinetically move the fluid with a longitudinal electric field. To modulate the EOF velocity, the technique of field-effect flow control (FEFC) is employed. In FEFC, a second electric field is applied through the microchannel wall to influence the surface charge at the fluid-microchannel interface for independent control of the EOF. Presented in this work is the first demonstration of FEFC in a polymer network. The microchannel walls were composed of Parylene C (1 - 2 um thick), which is an inexpensive, chemical vapor deposited polymer. In this work, FEFC modulated the EOF velocity from 240% to 60% of its original value in a microchannel that was 40 um in height, 100 um in width, and 2 cm long. The next research phase integrated FEFC technology into microfluidic networks with microchannels in the second and third dimensions. At the T-intersection of three microchannels, FEFC established different EOF pumping rates in the two main microchannels. The different flow rates induced pressure pumping in the third, field-free microchannel with ultra-low flow rate control (-2.0 nL/min to 2.0 nL/min). Moreover, adjusting the secondary electric fields enabled bi-directional flow control for the induced pumping in the 2D and 3D field-free microchannels. To improve the microfluidic networks, an electro-fluid flow model was developed to describe the induced pressure and FEFC phenomenon. The model closely predicted the experimentally obtained flow rates in the field-free microchannel. Additionally, the study of multiple gate electrodes along the same microchannel showed that FEFC has only a local effect over the EOF, but revealed that position and size of the electrodes influence the EOF control in the microfluidic network

Nanotechnology for Cell–Substrate Interactions

In the pursuit to understand the interaction between cells and their underlying substrates, the life sciences are beginning to incorporate micro- and nanotechnology-based tools to probe and measure cells. The development of these tools portends endless possibilities for new insights into the fundamental relationships between cells and their surrounding microenvironment that underlie the physiology of human tissue. Here, we review techniques and tools that have been used to study how a cell responds to the physical factors in its environment. We also discuss unanswered questions that could be addressed by these approaches to better elucidate the molecular processes and mechanical forces that dominate the interactions between cells and their physical scaffolds

The consequence of substrates of large- scale rigidity on actin network tension in adherent cells

International audienceThere is compelling evidence that substrate stiffness affects cell adhesion as well as cytoskeleton organization and contractile activity. This work was designed to study the cytoskeletal contractile activity of cells plated on microposts of different stiffness using a numerical model simulating the intracellular tension of individual cells. We allowed cells to adhere onto micropost substrates of various rigidities and used experimental traction force data to infer cell contractility using a numerical model. The model discriminates between the influence of substrate stiffness on cell tension and shows that higher substrate stiffness leads to an increase in intracellular tension. The strength of this model is its ability to calculate the mechanical state of each cell in accordance to its individual cytoskeletal structure. This is achieved by regenerating a numerical cytoskeleton base

Magnetic microposts as an approach to apply forces to living cells

Cells respond to mechanical forces whether applied externally or generated internally via the cytoskeleton. To study the cellular response to forces separately, we applied external forces to cells via microfabricated magnetic posts containing cobalt nanowires interspersed among an array of elastomeric posts, which acted as independent sensors to cellular traction forces. A magnetic field induced torque in the nanowires, which deflected the magnetic posts and imparted force to individual adhesions of cells attached to the array. Using this system, we examined the cellular reaction to applied forces and found that applying a step force led to an increase in local focal adhesion size at the site of application but not at nearby nonmagnetic posts. Focal adhesion recruitment was enhanced further when cells were subjected to multiple force actuations within the same time interval. Recording the traction forces in response to such force stimulation revealed two responses: a sudden loss in contractility that occurred within the first minute of stimulation or a gradual decay in contractility over several minutes. For both types of responses, the subcellular distribution of loss in traction forces was not confined to locations near the actuated micropost, nor uniformly across the whole cell, but instead occurred at discrete locations along the cell periphery. Together, these data reveal an important dynamic biological relationship between external and internal forces and demonstrate the utility of this microfabricated system to explore this interaction. Supporting materials: http://www.pnas.org/cgi/content/full/0611613104/DC

SARS-CoV-2 Infects Human Pluripotent Stem Cell-Derived Cardiomyocytes, Impairing Electrical and Mechanical Function

COVID-19 patients often develop severe cardiovascular complications, but it remains unclear if these are caused directly by viral infection or are secondary to a systemic response. Here, we examine the cardiac tropism of SARS-CoV-2 in human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) and smooth muscle cells (hPSC-SMCs). We find that that SARS-CoV-2 selectively infects hPSC-CMs through the viral receptor ACE2, whereas in hPSC-SMCs there is minimal viral entry or replication. After entry into cardiomyocytes, SARS-CoV-2 is assembled in lysosome-like vesicles and egresses via bulk exocytosis. The viral transcripts become a large fraction of cellular mRNA while host gene expression shifts from oxidative to glycolytic metabolism and upregulates chromatin modification and RNA splicing pathways. Most importantly, viral infection of hPSC-CMs progressively impairs both their electrophysiological and contractile function, and causes widespread cell death. These data support the hypothesis that COVID-19-related cardiac symptoms can result from a direct cardiotoxic effect of SARS-CoV-2

Mechanobiology of Platelets: Techniques to Study the Role of Fluid Flow and Platelet Retraction Forces at the Micro- and Nano-Scale

Coagulation involves a complex set of events that are important in maintaining hemostasis. Biochemical interactions are classically known to regulate the hemostatic process, but recent evidence has revealed that mechanical interactions between platelets and their surroundings can also play a substantial role. Investigations into platelet mechanobiology have been challenging however, due to the small dimensions of platelets and their glycoprotein receptors. Platelet researchers have recently turned to microfabricated devices to control these physical, nanometer-scale interactions with a higher degree of precision. These approaches have enabled exciting, new insights into the molecular and biomechanical factors that affect platelets in clot formation. In this review, we highlight the new tools used to understand platelet mechanobiology and the roles of adhesion, shear flow, and retraction forces in clot formation